A conventional surface acoustic wave (SAW) filter has been used extensively for communication equipment as a RF- and IF-stage filter within the reception and transmission circuits. Along with the recent development of mobile communication towards becoming digitalized, a digital mobile telephone or a digital cordless telephone has been intensively developed. In the communication system of these devices, a phase as well as an amplitude convey information, so that it is essential for a filter used for an IF-stage that the filter not only has excellent amplitude characteristics but also is flat in group delay deviation characteristics. Furthermore, this filter is required to have excellent selectivity characteristics for distinguishing a signal of a neighboring channel from a desired signal, for which steep out-of-band rejection characteristics are needed. In addition, along with the recent miniaturization of a set to attain higher mounting density, coupling or interference between components caused by lack in grounding strength and in screening has become a problem. Therefore, a balanced type circuit for controlling these influences has been rapidly developed.
Well-known conventional SAW filters which can be used for the IF-stage are a transversal type SAW filter and two types of SAW resonator filters which are a longitudinally coupled resonator filter and a transversely coupled resonator filter. The transversal type SAW filter has excellent group delay deviation characteristics. However, the insertion loss and the size are big, and in addition, the out-of-band rejection is poor. On the other hand, the SAW resonator filters have excellent out-of-band rejection characteristics and are small in insertion loss and size, but the group delay deviation characteristics are inferior to that of the transversal type SAW filter. In addition, the longitudinally coupled resonator type is characterized by the large spurious present on the close and high side of a pass band, while the transversely coupled resonator type is characterized by having extremely narrow-band pass characteristics. As a conventional IF filter of mobile communication, the transversely coupled resonator type SAW filter was used commonly which had a compact size and excellent out-of-band rejection characteristics.
A conventional transversely coupled resonator type SAW filter will be explained with reference to FIG. 24.
FIG. 24 shows an electrode pattern of a conventional transversely coupled resonator type SAW filter. Referring to FIG. 24, reference numeral 241 represents a monocrystal piezoelectric substrate on which an electrode pattern is formed to generate a surface acoustic wave. 242a is an inter-digital transducer (IDT) electrode which is disposed with reflectors 242b and 242c on both sides to form an energy trapping type SAW resonator. The same type of SAW resonator is formed by an IDT electrode 243a and reflectors 243b, 243c. When the above-mentioned two resonators are closely disposed to each other, an acoustic coupling occurs between the two resonators, thereby constructing a SAW resonator filter of the first stage. A SAW resonator filter of the second stage is constructed in the same manner as mentioned above by means of IDT electrodes 244a, 245a and reflectors 244b, 244c, 245b, and 245c. These two stages of SAW resonator filters are concatenately connected electrically through an electrode pattern 246 to comprise a multistage connected SAW filter.
In case of the SAW filter constructed above, the mode frequencies of the two different surface acoustic waves to be excited on the surface of the piezoelectric substrate are determined through an electrode overlap width of the IDT electrode and through a distance between the two closely disposed SAW resonators, thereby fixing the pass band width of the filter. However, this filter is characterized by its extremely narrow-band width to be achieved, so the above-noted structure of FIG. 24 can realize a fractional band width (band width standardized by the center frequency of a filter) of about 0.1% at the very most for the filter. In addition, since input-output impedance characteristics depend on the size of the above-noted IDT electrode finger overlap width, it is difficult to achieve optional impedance. Furthermore, the electrode structure of FIG. 24 can not achieve a balanced input-output due to the fact that the electrode fingers of IDT electrodes 242a, 245a are grounded on one side.
In order to keep step with the digitalization mentioned above, a flat band in group delay deviation characteristics is required to be broadened by broadening pass characteristics.
Furthermore, a balanced input-output needs to be attained. In the conventional method, an elongation coil was inserted between stages of a filter and a ground when the band needed to be broadened. A connection with surrounding circuits was attained by adding matching circuits at input-output stages.
This conventional structure, however, had a defect in that the circuit itself was large due to an increased number of components, since elongation coils or matching circuit elements were connected as the external circuits. At the same time, both the differences in and coupling of these elements affected the filter characteristics negatively, and furthermore, the input-output was unbalanced.